Types of sets used in World Championship

Yes, but from what I understand both FIDE and the USCF allow smaller and larger pieces (within limits) as long as the piece to square size ratio is acceptable. But as DGT only manufactures eboards with 55 mm squares all the top games are effectively limited to approximately 3.75" sets.

What size are the pieces that are used with the DGT board? From what I know, the board has 55mm squares and I've read in DGT timeless pieces that they have 36mm, but there is no way that 36mm pieces look like that on a 55 mm board.

I have the same pieces and board size at home and the pieces have a lot of space on the square, compared to the videos, on which the pieces are very "fat" and they occupy a much more space.

I think the reason people use timeless is beause the pieces are more duriable and less sharp corner to be dent. As well, the walnut board with red rosewood are moderate contract colors for vision wise, so good to play long time match game.

I think the Timeless set is about as "vanilla" as a set can be, with its clean pleasant looking Knight without disturbing detail. The rest of the pieces are also completely harmless in their design. As such it's an ideal playing set, and that should account on its popularity.